This disclosure pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Microlithography is a key technology used in the production of microelectronic devices such as semiconductor integrated circuits, writing and pickup heads used in magnetic memory devices, displays, and micromachines, for example. More specifically, the disclosure pertains to reticles as used in charged-particle-beam (CPB) microlithography, especially to so-called xe2x80x9cstencilxe2x80x9d reticles and to methods for defining a pattern on one or more such reticles.
As microelectronic devices have reached ever-higher levels of integration, it has become increasingly difficult to use optical microlithography for forming circuits and other patterns on the surface of a substrate. Consequently, xe2x80x9cnext generationxe2x80x9d lithography (NGL) technology currently is under intensive development. A promising NGL technology is the so-called xe2x80x9ccharged-particle-beamxe2x80x9d (CPB) lithography technology that utilizes a charged particle beam such as an electron beam for making pattern exposures. CPB lithography offers prospects of substantially greater pattern resolution for reasons similar to the reasons for which electron microscopy yields better pattern resolution than optical microscopy.
Aberrations, distortion, and the like in a CPB optical system make it impossible for a wide pattern area (e.g., corresponding to the area of a single xe2x80x9cchipxe2x80x9d) to be transfer-exposed in a single exposure xe2x80x9cshot.xe2x80x9d Consequently, patterns must be a transfer-exposed in multiple steps each pertaining to a respective portion of the pattern. More specifically, a pattern for a layer of a chip is divided into multiple exposure units (typically called xe2x80x9csubfieldsxe2x80x9d) each defining a respective portion of the pattern. The pattern portions defined in the subfields are transferred individually to a lithographic substrate, on which respective images of the pattern portions are formed adjacent each other in a manner that achieves xe2x80x9cstitchingxe2x80x9d of the pattern portions together to form the complete pattern for a chip.
This manner of exposure is diagrammed in FIGS. 12 and 13. FIG. 12 depicts a lithographic substrate (xe2x80x9cwaferxe2x80x9d) on which multiple chip patterns have been formed. The pattern for a single chip is divided into multiple stripes (four stripes are shown), and each stripe contains multiple subfields arranged in a rectilinear array of rows and columns. The corresponding pattern as defined on the reticle (not shown in FIG. 12) is similarly divided. FIG. 13 depicts an actual exposure performed using a charged particle beam. The reticle, situated upstream in the figure, is mounted on a reticle stage (not shown). The substrate, situated downstream in the figure, is mounted on a wafer stage (not shown). The reticle stage and a wafer stage move the reticle and substrate in the directions shown at respective constant velocities. Note that the stage-movement directions are opposite each other. In the figure, an xe2x80x9cillumination beamxe2x80x9d is shown in position to begin exposure of a row of subfields in a stripe on the reticle. Similarly, the xe2x80x9cpatterned beam,xe2x80x9d propagating downstream of the reticle, is shown in position to begin imaging of the subfields in a corresponding row in a corresponding stripe on the substrate. The illumination beam is directed to the desired subfield on the reticle by an xe2x80x9cillumination-optical systemxe2x80x9d and the patterned beam is directed to the desired subfield on the substrate by a xe2x80x9cprojection-optical system.xe2x80x9d
As exposure of the stripe progresses, the respective stages move the reticle and substrate in the directions shown while deflectors in the illumination-optical system and projection-optical system deflect the illumination beam and patterned beam, respectively, laterally as shown. Note that the directions of beam deflection are substantially perpendicular to the respective directions of stage movement. Thus, the subfields in each row and the rows of subfields in each stripe are exposed in sequential order. After exposure of a row of subfields is complete, exposure of the subsequent row begins, but with a direction of beam deflection that is opposite the direction of beam deflection used in the just-completed row. In other words, the subfields in the rows are exposed in a raster manner, which is time-efficient and thus maximizes throughput.
Since the respective pattern portion in each subfield is exposed in a respective xe2x80x9cshot,xe2x80x9d throughput is greater using the depicted technique than was obtained previously using conventional xe2x80x9cdirect writing,xe2x80x9d xe2x80x9ccell projection,xe2x80x9d and xe2x80x9ccharacter projectionxe2x80x9d techniques of performing CPB microlithography.
Further with respect to a reticle for use in CPB microlithography as shown in FIGS. 12 and 13, the subfields are defined in respective membrane portions of the reticle. The membrane portions are not contiguous with each other but rather are divided from one another by support struts. The struts provide substantial mechanical strength to the membrane portions. In a representative membrane portion, the respective subfield (which is the portion of the membrane portion actually defining the respective portion of the pattern) is surrounded by a non-patterned xe2x80x9cskirt.xe2x80x9d The skirt is the region of the membrane portion in which the edges of the illumination beam fall whenever the respective subfield is being illuminated by the illumination beam for exposure. Thus, as each subfield is exposed, only the respective pattern portion is transferred to the substrate.
Before CPB lithographic exposure can be performed, the pattern to be exposed on the substrate must be defined on the reticle. CPB microlithography reticles can be classified broadly into two types. A first type is the so-called xe2x80x9cstencilxe2x80x9d reticle, in which pattern elements are defined as respective voids (through-holes) in a reticle membrane that is 1 to 5 xcexcm thick. The voids are highly transmissive to the incident illumination beam with substantially no forward scattering, whereas intervening portions of the reticle membrane cause substantial forward scattering of charged particles in the illumination beam. A second type of reticle is the so-called xe2x80x9ccontinuous-membranexe2x80x9d reticle, in which pattern elements are defined as respective openings in a highly scattering layer formed on a continuous, relatively low-scattering membrane layer. In other words, the openings have relatively high transmissivity to the incident beam, and regions occupied by the highly scattering layer have relatively low transmissivity to the incident beam. xe2x80x9cTransmissivityxe2x80x9d as referred to here takes into account the degree of forward-scattering. A relatively low transmissivity means not only that a portion of the beam may be absorbed but also that portions of the incident beam transmitted through the respective region of the reticle are forward scattered sufficiently so as not to reach the substrate.
With either type of reticle, a xe2x80x9ccontrast aperturexe2x80x9d is situated at a beam crossover of the projection-optical system. As the patterned beam (containing both scattered and relatively non-scattered charged particles) propagates from the reticle to the substrate, the contrast aperture blocks the relatively highly scattered charged particles of the beam, thereby preventing such particles from reaching the substrate. Relatively low-scattered and non-scattered charged particle are transmitted by the scattering aperture and focused on the substrate. Thus, the image formed on the substrate is provided with contrast.
A continuous membrane reticle as summarized above exhibits problems with thermal absorption of incident charged particles of the illumination beam. As a result, a continuous membrane reticle tends to exhibit chromatic aberration. A stencil reticle, on the other hand, does not exhibit these problems, which is regarded as a desirable aspect of stencil reticles because better pattern resolution usually can be obtained with them.
However, stencil reticles are prone to certain problems if the reticle defines xe2x80x9cdonutxe2x80x9d-shaped (annular) pattern features or linear pattern elements longer than a certain length. For example, if a pattern element is annular, then the membrane portion surrounded by the annular opening in the membrane is unsupported. Annular pattern elements must be divided into at least two complementary portions requiring separate exposure onto the substrate. Hence, at least two separate exposures are required to expose annular pattern elements. The projected portions must be positioned on the substrate such that the portions are xe2x80x9cstitched togetherxe2x80x9d properly to form the complete annular element of the pattern. This manner of dividing certain pattern elements into complementary portions is termed xe2x80x9ccomplementary division.xe2x80x9d
Complementary division also is required whenever linearly extended pattern elements are defined as respective openings in the membrane of a stencil reticle. Linearly extended elements (i.e., having a length greater than a threshold length xe2x80x9cLxe2x80x9d) are flanked by membrane regions that are weak and easily subject to deformations such as bending, sagging, and the like. (Preventing bending and sagging of these portions of the membrane would require application of substantial tensile force to the membrane, causing undesired changes to the shape of the element-defining apertures in the membrane.) Hence, these pattern elements also must be subject to complementary division.
On a stencil reticle, an annular pattern element requiring complementary division is relatively easy to identify from the contour of the pattern element. In contrast, linearly extended and other pattern elements defined by through-holes that excessively weaken the membrane or cause reticle distortions are problematic because reliable identification of such elements is difficult using conventional methods. It also is difficult, using conventional methods, to determine the manner in which such pattern elements should be divided into complementary pattern-element portions so as to eliminate reticle distortion.
The criterion for determining the threshold length L of a pattern element that may require complementary division depends upon the process used for fabricating the stencil reticle, the residual stress in the reticle membrane, the Young""s modulus of the membrane, and other factors. Generally, L (termed herein the xe2x80x9cdivision criterionxe2x80x9d) is several tens of film on the reticle. If a pattern element is linearly extended in only one dimension, then the element readily may be divided into multiple complementary portions each having a length equal to or less than L. However, if the pattern element extends in two dimensions or is a large-area element (e.g., a wiring pad) having both a long side xe2x80x9caxe2x80x9d and short side xe2x80x9cbxe2x80x9d that are each longer than L, then complementary division conventionally is performed as shown in FIG. 14.
FIG. 14 depicts a square pattern element 60 having a first side of length xe2x80x9caxe2x80x9dxe2x89xa7L and a second side of length xe2x80x9cbxe2x80x9dxe2x89xa7L. The first-side length xe2x80x9caxe2x80x9d is divided into four portions, and the second-side length xe2x80x9cbxe2x80x9d is divided into four portions, thereby forming 16 small, square pattern-element portions 61 each having a length and width of less than L.
In the example depicted in FIG. 14, the complementarily divided pattern element 60 is defined in respective pattern-element portions on three stencil reticles (reticle 62, reticle 63, and reticle 64) as respective arrays of pattern-element portions 61. In each reticle 62, 63, 64 the constituent pattern-element portions 61 are arranged diagonally relative to each other, and each diagonal row is separated from the other in each reticle by two intervening diagonal rows. For example, reticle 62 defines six pattern-element portions 61 (each as a respective aperture in the membrane): one at the lower left corner, one at the upper right corner, and a diagonal row of four extending from the upper left corner to the lower right corner. This arrangement of apertures does not cause a problem with stress in the normal direction of gravity on the reticle membrane.
Unfortunately, however, in this example complementary division of the pattern element 60 requires three exposures to complete transfer of the pattern element 60 to the substrate, one respective exposure for each complementarily divided reticle 62, 63, 64. Other pattern elements may require a larger number of complementary divisions (and thus a larger number of exposures per element). These many exposures produce correspondingly large reductions in throughput, which are difficult to impossible to tolerate in a modern wafer-fabrication facility.
Also, in a stencil reticle, if a relatively small pattern element is present near a relatively large-area pattern element, stress-caused membrane deformation around the relatively large-area pattern element adversely affects the relatively small pattern element, causing positional displacement of the small element. This phenomenon also occurs with large-area pattern elements that are not so large as to require complementary division (except under special conditions such as when the membrane stress is sufficiently close to 0 Pa). Therefore, certain instances may require complementary division, even though the subject pattern element(s) would not require complementary division based solely on the size of the pattern element(s).
In view of the shortcomings of the prior art as summarized above, the invention provides, inter alia, methods for complementarily dividing certain linearly extended and large-area pattern elements (or other pattern elements requiring complementary division) in stencil reticles such that throughput is not reduced significantly.
According to a first aspect of the invention, stencil reticles are provided that comprise a self-supporting reticle membrane defining apertures having respective opening profiles corresponding to respective pattern elements defined by the reticle. In an embodiment of such a reticle, among the pattern elements defined by the reticle, a linearly extended pattern element having a width less than a predetermined division criterion L and a length equal to or greater than L is complementarily divided into respective linear pattern-element portions each having a respective length less than L. Also, a large-area pattern element, having both length and width equal to or greater than L, is complementarily divided into respective linear pattern-element portions each having a respective width less than L and a respective length equal to or greater than L.
In general, the required pattern accuracy on a reticle intended for use in the 70-nm node is, for example, 50 nm. The division criterion L noted above is usually several tens of xcexcm on the reticle. Hence, L is about two orders of magnitude larger than the pattern accuracy required on the reticle. In practice, high linewidth accuracy and precision usually are not required for large-area pattern elements such as wiring pads. Also, some positional displacement is allowable for large-area pattern elements such as wiring pads. Therefore, overall pattern-position accuracy is largely unaffected if large-area pattern elements are divided into linear pattern-element portions having lengths equal to or greater than L and widths less than L.
Desirably, in the stencil-reticle embodiment summarized above, the respective linear pattern-element portions formed from complementarily dividing the large-area pattern element each comprise at least one overlap region extending along an edge (shared with an edge of an adjacent pattern-element portion of the large-area pattern element when the large-area pattern element is projection-transferred from the reticle to a substrate) of the pattern-element portion. If the pattern elements exhibit any bending or the like on the reticle, the overlap regions reduce exposure anomalies that otherwise would occur at the bends when images of the pattern-element portions are projected onto and stitched together on the lithographic substrate.
In another embodiment of a stencil reticle, among the pattern elements defined by the reticle, a linearly extended pattern element having a width less than the predetermined division criterion L and a length equal to or greater than L is complementarily divided into respective linear pattern-element portions each having a respective length less than L. A large-area pattern element, having both length and width equal to or greater than L, is not complementarily divided.
Large-area pattern elements of which the sides (on the reticle) are at least, for example, 10xc2x7L in length often do not require high accuracy as defined on the reticle, so complementary division of such elements is unnecessary. Hence, in yet another embodiment of a stencil reticle, among the pattern elements defined by the reticle, linearly extended pattern elements having width less than the predetermined division criterion L and length equal to or greater than L are complementarily divided into respective linear pattern-element portions each having a respective length less than L. But, large-area pattern elements having both length and width equal to or greater than 10xc2x7L are not complementarily divided.
According to another aspect of the invention, charged-particle-beam (CPB) microlithography methods are provided. In an embodiment of such a method, a pattern is defined on a stencil reticle comprising a self-supporting reticle membrane defining apertures having respective opening profiles corresponding to respective elements of the pattern. At least a portion of the reticle is illuminated with a CPB illumination beam (e.g., electron beam). An image of the illuminated portion of the reticle is projected and focused onto a lithographic substrate. The illumination and projection steps are repeated as required to complete projection-transfer of the pattern to the lithographic substrate. The pattern is defined on the reticle such that, among the pattern elements defined by the reticle, a linearly extended pattern element having a width less than the predetermined division criterion L and a length equal to or greater than L is complementarily divided into respective linear pattern-element portions each having a respective length less than L. Also, a large-area pattern element, having both length and width equal to or greater than L, is complementarily divided into respective linear pattern-element portions each having a respective width less than L and a respective length equal to or greater than L.
Another embodiment of a CPB microlithography method comprises the general pattern-defining, illumination, and projection steps summarized above. However, instead of the specific pattern-defining details summarized above, the pattern is defined on the reticle such that, among the pattern elements defined by the reticle, a linearly extended pattern element having a width less than the predetermined division criterion L and a length equal to or greater than L is complementarily divided into respective linear pattern-element portions each having a respective length less than L. But, a large-area pattern element having both length and width equal to or greater than at least 1xc2x7L is not complementarily divided.
According to yet another aspect of the invention, methods are provided for configuring a stencil reticle. An embodiment of such a method comprises, with respect to pattern elements (defined by the reticle) that are linearly extended with a width less than the predetermined division criterion L and a length equal to or greater than L, complementarily dividing the pattern elements into respective linear pattern-element portions each having a respective length less than L. Also, with respect to large-area pattern elements, having both length and width equal to or greater than L, such pattern elements are complementarily divided into respective linear pattern-element portions each having a respective width less than L and a respective length equal to or greater than L. The pattern-element portions can be divided into at least a first group and a second group, wherein the first group is defined on a first stencil reticle, and the second group is defined on a second stencil reticle. Alternatively, the first and second groups can be defined on first and second regions, respectively, of the same stencil reticle. Furthermore, the pattern-element portions can be configured with overlap regions as summarized above.
In another embodiment of the methods for configuring a stencil reticle, with respect to linearly extended pattern elements having a width less than the predetermined division criterion L and a length equal to or greater than L, the pattern elements are complementarily divided into respective linear pattern-element portions each having a respective length less than L. But, large-area pattern elements, having both length and width equal to or greater than at least 1xc2x7L, are not complementarily divided.
In yet another embodiment of the methods for configuring a stencil reticle, the subject reticle includes a self-supporting reticle membrane exhibiting blocking behavior to an incident charged particle beam, and the membrane defines non-blocking regions of the membrane having respective profiles corresponding to respective pattern elements defined by the reticle. With respect to a pattern element in which the respective non-blocking region surrounds at least three sides of a protruding (xe2x80x9cconvexxe2x80x9d) blocking region having a width W and length H, a size error E of the pattern element is calculated from the length H and width W of the convex blocking region and from a length S of a side of the respective non-blocking region into which the blocking region protrudes. If the size error E is greater than a predetermined tolerance value T, then the pattern element is split into at least two complementary pattern-element portions. In this method embodiment, the size error E can be expressed as:
E={(k1xc2x7H+k2xc2x7S)2+[k3xc2x7W/2]2}1/2
wherein k1, k2, and k3 are predetermined constants. The at least two complementary pattern-element portions can be defined on separate reticles or on separate regions of a single reticle.
In this regard, based upon examinations of the change in the shape of a pattern element caused by tensile force applied to a stencil reticle, it was discovered that, whenever an unexposed region on the lithographic substrate (corresponding to a respective xe2x80x9cblocking regionxe2x80x9d on the reticle) includes a xe2x80x9cprotrudingxe2x80x9d (xe2x80x9cconvexxe2x80x9d or xe2x80x9cpeninsularxe2x80x9d portion) portion bounded on three sides by the respective exposed region, deformation of the pattern element caused by tensile stress is particularly great at the distal end of the protruding portion. It also was discovered that the magnitude of deformation is related to the length H and width W of the protruding portion and to the length S of the side of the pattern element from which the protruding portion extends, as expressed in the equation above.
Hence, at the time of determining the respective shapes of elements of a pattern to be transferred to the substrate, a determination is made as to whether an unexposed region on the substrate will include a protruding portion as summarized above. If the determination reveals such a feature associated with a pattern element, then the size error E of the pattern element is calculated, relative to the as-designed pattern element, using the equation noted above. If E greater than T, then the element is divided into complementary pattern-element portions. This allows the accuracy of the pattern element, as projection-transferred to the substrate, to be kept within an allowable range.
If the pattern element, including a protruding portion as summarized above, loses the protruding portion as a result of the pattern being divided into subfields, then there is no need to divide the pattern element in a complementary manner as summarized above.
Deformation of a pattern element is determined not only by H, W, and S, but also by the material of the reticle, the tension of the reticle, the size of the reticle, the demagnification ratio by which the pattern is transferred from the reticle to the substrate, and the reticle-manufacturing process. The term xe2x80x9creticle sizexe2x80x9d as used herein denotes the outside dimensions of the reticle (e.g., whether it was made from a 300-mm wafer or from a 200-mm wafer). Physical characteristics of the reticle also include the respective dimensions of the subfields, the membrane portions within the subfields, the arrangement subfields and membrane portions thereof, as well as the number, width, and arrangement of support struts. These factors were varied and subjected to finite-element and regression analyses to examine the relationships of these factors to the size error E of individual pattern elements relative to corresponding xe2x80x9cas-designedxe2x80x9d pattern elements. The equation noted above was found to approximate these relationships closely. In the equation the constants k1, k2, and k3 have respective values determined by the material of the reticle, the reticle tension, the reticle size, the demagnification ratio, and the reticle-manufacturing process. So long as these parameters remain unchanged, these constants can be used regardless of the shapes of the elements of the pattern.
In yet another embodiment of the methods for configuring a stencil reticle, with respect to a pattern element in which the respective non-blocking region surrounds at least two sides of a blocking region having width L1 and length L2, a size error E of the pattern element is calculated from the width L1 and length L2 of the blocking region. If E greater than T, then the pattern element is divided into at least two complementary pattern-element portions. The size error E can be expressed as:
E=k4xc2x7L1+k5xc2x7L2
wherein k4 and k5 are predetermined constants. Again, the at least two complementary pattern-element portions can be defined on separate reticles or on separate regions of a single reticle.
By examining changes in pattern-element shape caused by tensile stress in a stencil reticle, it was discovered that, whenever a non-blocking region has two sides bounded by a protruding (xe2x80x9cconvexxe2x80x9d) blocking region (i.e., the blocking region is configured as a two-sided protrusion into the respective non-blocking region), tensile deformation of the respective pattern element is especially great at the distal portion of the protrusion. It also was discovered that the magnitude of deformation is related to the lengths L1 and L2 of the sides of the protrusion, as expressed in the equation above.
Hence, at the time of determining the respective shapes of elements of a pattern to be transferred to the substrate, a determination is made as to whether an unexposed region on the substrate will include a protruding portion as summarized above. If the determination reveals such a feature associated with a pattern element, then the size error E of the pattern element is calculated, relative to the as-designed pattern element, using the equation noted above. If E greater than T, then the element is divided into complementary pattern-element portions.
If the pattern element, including a protruding portion as summarized above, loses the protruding portion as a result of the pattern being divided into subfields, then there is no need to divide the pattern element in a complementary manner as summarized above.
Deformation of a pattern element is determined not only by L1 and L2, but also by the material of the reticle, the tension of the reticle, the reticle size, the demagnification ratio, and the reticle-manufacturing process. These factors were varied and subjected to finite-element and regression analyses to examine the relationships of these factors to the size error E of individual pattern elements relative to corresponding xe2x80x9cas-designedxe2x80x9d pattern elements. The equation noted above as found to approximate these relationships closely. In the equation the constants k4 and k5 have respective values determined by the reticle material, the reticle tension, the reticle size, the demagnification ratio, and the reticle-manufacturing process. So long as these variables remain unchanged, these constants can be used regardless of the shapes of the elements of the pattern.
According to another embodiment of a CPB microlithography method, the pattern is defined, illuminated, and projected as generally summarized above. The pattern is defined on the reticle such that, among the pattern elements defined by the reticle, and with respect to a pattern element in which a respective non-blocking region surrounds at least three sides of a blocking region having width W and length H, the size error E is calculated from H and W and from the length S of a side of the respective non-blocking region into which the blocking region extends. If E greater than T, then the pattern element is divided into at least two complementary pattern-element portions. The size error E is expressed as:
E={(k1xc2x7H+k2xc2x7S)2+[k3xc2x7W/2]2}1/2
In yet another embodiment of a CPB microlithography method, the pattern is defined, illuminated, and projected as generally summarized above. The pattern is defined on the reticle such that, among the pattern elements defined by the reticle, with respect to a pattern element in which a respective non-blocking region surrounds at least two sides of a blocking region having width L1 and length L2, the size error E is calculated from L1 and L2. If E greater than T, then the pattern element is divided into at least two complementary pattern-element portions. The size error E is expressed as:
E=k4xc2x7L1+k5xc2x7L2
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.